1. Field of the Invention
The present invention relates to an apparatus and method for manufacturing an optical fiber preform using a so-called outside vapor-deposition method.
2. Description of the Related Art
Generally, in an apparatus for manufacturing an optical fiber preform on the basis of a manufacturing method which is called an outside vapor-deposition method, a rod-shaped target member is rotated by a glass lathe or the like while both ends thereof are held by the glass lathe, and glass particles generated in the flame of a flaming burner used for generating glass particles are deposited on the periphery of the target member. The target member may be removed in the subsequent process or may serve as a silica-based glass rod functioning as a core of an optical fiber obtained in the subsequent process.
A flame hydrolysis reaction or the like occurs in the flame by introducing a glass source material gas, a combustion gas, and a supporting gas into the flame of the burner and, glass particles, such as SiO2 particles, are thereby generated. The glass particles are deposited on the periphery of the rotating target member as described above.
The deposition process is performed while the flaming burner used for generating glass particles is traversed in the axis direction of the target member to form a glass particle deposition layer on the periphery of the target member. When the deposition layer reaches a predetermined weight, the deposition process is completed.
The glass particle deposited body, which is a complex of the target member and the glass particle deposition layer formed in this way is subsequently heated in a high-temperature furnace, the glass particle deposition layer is sintered and vitrified into transparent glass to obtain an optical fiber preform.
In the apparatus for manufacturing an optical fiber preform, deposition of glass particles may be carried out by sequentially performing traverse of a plurality of flaming burners used for generating glass particle in a single direction. In this case, one burner performs glass particle generation and deposition in only the period from the deposition start point to the deposition completion point, and then returns to the deposition start point along a path deviating from the traverse path so as not to interfere with the other burners which perform deposition of the glass particles while being traversed. In the return period, the flame of the burner needs to be as small as possible. For this reason, in the related art, the flow rate of combustion gas and supporting gas is reduced as much as possible, or the flow rate of the combustion gas is significantly reduced while closing a supporting gas valve.
However, when the flow rate of the combustion gas and the supporting gas is reduced as much as possible in the return period from the deposition completion point to the deposition start point as described above, the vicinity of the nozzle of the flaming burner used for generating glass particles is burned and the nozzle tip glows, thereby significantly reducing the lifespan of the burner.
With respect to the foregoing problems, for example, the following methods have been proposed: a method (for example, see Japanese Unexamined Patent Application, First Publication No. H04-170336: hereinafter referred to as Patent Document 1) of stopping the introduction of oxygen in the return period; and a method (for example, see Japanese Unexamined Patent Application, First Publication No. H04-175239: hereinafter referred to as Patent Document 2) of supplying a purge gas into an oxygen nozzle in the return period.
In order to prevent the deterioration of the tip of the oxygen nozzle, for example, the following methods have been proposed: a method (for example, see Japanese Unexamined Patent Application, First Publication No. H11-79774: hereinafter referred to as Patent Document 3) of using a mixed gas of hydrogen and inert gas or nitrogen; a method (for example, see Japanese Unexamined Patent Application, First Publication No. H06-247722: hereinafter referred to as Patent Document 4) which reduces the thickness of a nozzle to 1 mm or less to increase the flow rate of gas; and a method (for example, see Japanese Unexamined Patent Application, First Publication No. S59-232933: hereinafter referred to as Patent Document 5) in which a sealing layer is provided on the periphery of an oxygen gas nozzle.
In the above-mentioned Patent Documents, oxygen corresponds to “supporting gas”, hydrogen corresponds to “combustion gas”, and inert gas or nitrogen corresponds to “purge gas”.
However, in the methods disclosed in Patent Document 1 and Patent Document 2, at the moment when the flow rate of gas is changed, that is, the flow rate of gas is reduced, the nozzle temporarily becomes a high temperature, and reiteration of this may cause deformation of the nozzle.
In the method disclosed in Patent Document 3, since deposition efficiency is greatly affected by manufacturing conditions, preferable results may not be achieved.
In the method disclosed in Patent Document 4, since the flow rate of gas is reduced during ignition, extinguishment, a pilot light, and the like, the nozzle glows. Further, when the thickness of the nozzle is equal to or less than 1 mm, the deformation of the nozzle due to the glow becomes significant, and thereby reducing the lifespan of the nozzle.
In the method disclosed in Patent Document 5, the deformation of the nozzle can be prevented, but the size of the burner increases and the structure of the burner becomes complicated since the sealing layer is provided, which is not preferable. In addition, the manufacturing accuracy of the burner is reduced, or deposition efficiency is reduced since the burner is excessively large.
The present invention has been made in view of the above-mentioned problems and a first object of the present invention is to provide a method for manufacturing an optical fiber preform using a flaming burner used for generating glass particles which is capable of preventing the deterioration of a nozzle tip due to glow without a reduction in deposition efficiency and is capable of effectively preventing the deterioration of the nozzle tip particularly when a mode change between the deposition mode and the non-deposition mode (pilot light state) is frequently reiterated.
A second object of the present invention is to provide an apparatus for manufacturing an optical fiber preform including a flaming burner used for generating glass particles, the apparatus having a simple structure being capable of preventing the deterioration of a nozzle tip due to glow, and being capable of increasing the lifespan of a burner even when a mode change between the deposition mode and the non-deposition mode (pilot light state) is frequently reiterated.